a) Field of the Invention
The present invention relates to a range finder for measuring a distance to a subject by focusing light beams propagated via different optical paths from the subject on a pair of photosensor arrays, calculating a correlation factor between an image on one photosensor array and another image on the other photosensor array by shifting the images relative to each other, and using a shift amount providing the highest correlation factor for determining the distance.
b) Description of the Related Art
Referring to FIGS. 34A and 34B, an example of a TTL (through the lens) range finder of a phase difference detection type according to a conventional technique will be described. FIG. 34A shows the structure of the range finder, and FIG. 34B shows an example of a processor circuit. A focus point detector for use with cameras will be described by way of example.
Light beams from a subject to be taken are converged by a taking lens 201, and via a film equivalent plane 202, reach a condenser lens 203 and a separator lens 204. The separator lens 204 divides the incident light beams into two light beam groups which are focused on a standard line sensor 205 and a reference line sensor 206. An image of a subject along an optical axis 208 of the taking lens 201 is separated into two images by the separator lens 204 and focused on the line sensors 205 and 206.
The line sensor 205 has p light receiving elements, and is used as a standard so that it is called a standard line sensor. The other line sensor 206 has q light receiving elements larger than p, and is called a reference line sensor. Signals are read from p light receiving elements of the line sensor 206 while changing the signal read phase, and compared to signals read from the standard line sensor 205 to detect a phase difference.
Signals detected from the standard and reference line sensors 205 and 206 are supplied to a processor circuit 207. The processor circuit 207 calculates a correlation factor to be described later while changing the read phase of a signal to be detected from the reference line sensor 206, and detects an extreme value of correlation factors to detect an in-focus position.
Another method of measuring a distance to a subject has been proposed in which light beams from a subject are not received by a taking lens 201 but directly received by a pair of lenses having the same characteristics placed in front of a reference line sensor and a standard line sensor.
FIG. 34B shows an example of the structure of the processor circuit 207. Analog signals from the standard and reference line sensors 205 and 206 are supplied to an A/D converter 209 to convert the analog signals into digital signals. The digital signals are temporarily stored into a RAM 211 under the control of a CPU 210. The digital signals in RAM 211 are thereafter read, and CPU 210 performs a correlation factor calculation to detect an extreme value of correlation factors, and generates an output signal representing a distance to a subject.
In the focus point detector shown in FIGS. 34A and 34B, electric charges accumulated in the photosensor are directly converted into a voltage which forms an analog detection signal. This analog signal is converted into a digital signal and temporarily stored in RAM 211. Thereafter, it is read for the calculation of a correlation factor.
The above-described range finder measures a distance to a subject only for an image focused on the standard line sensor 205. Generally, only a central image in a frame is formed on the standard line sensor to measure a distance. Therefore, if a main subject is at a position shifted from the frame center, the main subject becomes out of focus.
As a method of eliminating this disadvantage, a range finder has been proposed as disclosed in, for example, Japanese Patent Publication 3-67203, which can measure a distance to a subject not only at a frame central area but also at a frame peripheral area. This range finder has a standard line sensor longer than a conventional one to form a plurality of standard positions on the standard line sensor, to allow simultaneous measurements of a plurality of frame areas.
A conventional phase difference detection type range finder can measure a distance to a subject whose image is focused on the standard line sensor. Generally, the standard line sensor is designed so as to focus the image of a subject in the direction perpendicular to the line sensor surface. Therefore, this range finder mounted on a camera can measure the distance to a subject only at the frame central area.
If a main subject is at the position shifted from the frame central area, the distance to the main subject is measured by directing the camera toward the main subject, and thereafter the camera is directed to the original direction to depress the shutter.
The range finder disclosed in Japanese Patent Publication 3-67203 can measure the distance to a subject at the position shifted from the frame central area. However, this range finder cannot determine which area among a plurality of areas contains a main subject. As a result, the distance to the main subject cannot be calculated from information regarding a plurality of distances. In addition, since the areas capable of measuring distances are fixed, if a main subject is not in these fixed areas, the subject distance cannot be measured.
It is also necessary to provide correlation factor calculating circuits as many as the number of distance measuring areas. If a number of distance measuring areas are used, the circuits become complicated.
After electric charges accumulated in a photosensor are read and converted into a digital signal, there is no charge left in the photosensor. If a light amount is insufficient, it is necessary to apply light again to the photosensor to accumulate electric charges. In this case, a range finding time becomes long.